1. Field of the Invention
The present invention relates to the protection against electric overvoltages of telephone exchanges and, more particularly, of the interface between a telephone line and an exchange. Such an interface, currently designated in the art as a Subscriber Line Interface Circuit (SLIC), is generally comprised of a card which receives, for example, eight telephone lines.
Although the present invention is described hereafter within the framework of this application, it should be noted that the present invention provides a protection system that can also apply to the protection of one or several electric conductors connected to other electric or electronic circuits.
2. Discussion of the Related Art
FIG. 1 shows a conventional embodiment of an SLIC. Between conductors T and R (Tip and Ring) of a subscriber telephone line and SLIC interface 1, is positioned a circuit 2 of inverting relays, called ringing relays. The ringing relays act to switch the line between two operating modes. A first so-called "ringing" operating mode connects conductors T and R, respectively through relays I1 and I2, to a ringing generator 3 via lines 3a and 3b. In a second so-called "speech" operating mode (shown in FIG. 1), conductors T and R are connected via relays I1 and I2 to circuit 1 via lines E.sub.1 and E.sub.2.
SLIC circuit 1 manages the transistor between the two operating modes. Circuit 1 has a control output 4 issuing a two-state signal for controlling a block 5 operating relays I1 and I2. The control of block 5 is generally performed via a transistor Tc, the transmitter of which is connected to the ground, the collector of which is connected to a control input of block 5 and the base of which is connected to terminal 4 via a resistor Rc. Block 5 is supplied by a positive voltage V (generally around 5 volts). Alternatively, the assembly comprised of transistor Tc and resistor Rc can be integrated into SLIC circuit 1.
The usual protection of the SLIC circuit and of ringing generator 3 is provided by placing protection device 6 between the line and relay circuit 2, and by placing protection device 7 between SLIC circuit 1 and circuit 2.
Device 6 provides protection for the ringing generator 3 when relays I1 and I2 connect the line thereto (in ringing mode), by connecting one or the other of conductors T and R to the ground as soon as the line voltage overcomes a determined threshold, for example, a voltage of .+-.200 volts. The selection of the voltage threshold is determined by the operating mode of the ringing generator. In the ringing mode, an a.c. voltage called the ringing signal is applied to the line and has a peak value generally between 50 and 100 volts RMS. The function of device 6 is to suppress any voltage higher than the ringing peak voltage.
Protection device 7 has the function of protecting SLIC circuit 1 when the contacts of relay circuit 2 connect the line thereto (in speech mode, the position shown in FIG. 1). Device 7 acts to connect one or the other of connectors T or R to the ground as soon as the voltage on the line exceeds a determined threshold corresponding to the normal voltage level on the line. In the speech mode, circuit 1 is supplied by a negative voltage generally included between -30 and -70 d.c. volts (for example -48 volts) using the ground as a reference. This voltage is called "battery voltage". The inputs, respectively E1 and E2, of circuit 1 on the line side have to be protected from any positive voltage as well as from any voltage which is more negative than the battery voltage.
Protection devices 6 and 7 generally are so-called crowbar devices. An example implementation of a typical protection device and the current-voltage characteristic of the example protection device are respectively shown in FIGS. 2A and 2B.
FIG. 2A shows a conventional embodiment of a half-protection circuit of the crowbar type for one of conductors T or R. In FIG. 2A, only one half of protection device 6 has been shown since it has the same structure between each of conductors T or R and the ground.
For the portion shown in FIG. 2A (protection of conductor T), device 6 includes a first anode-gate thyristor Th1 mounted between conductor T and the ground, its cathode being connected to the ground. The gate of thyristor Th1 is connected to the cathode of an avalanche diode Z1, the anode of which is connected to the ground. When a positive overvoltage exceeding the avalanche voltage of diode Z1 occurs on conductor T, diode Z1 starts an avalanche and turns on thyristor Th1. Device 6 includes a second cathode-gate thyristor Th2 mounted between conductor T and the ground, its anode being connected to the ground. The gate of thyristor Th2 is connected to the ground via a second avalanche diode Z2 mounted with a biasing opposite to that of avalanche diode Z1 associated with thyristor Th1. Diode Z2 starts an avalanche when a negative overload exceeding its avalanche voltage occurs on conductor T and, thereby, thyristor Th2 turns on.
The avalanche voltages of diodes Z1 and Z2 are set to correspond to the circuit protection threshold voltages, for example, .+-.200 volts.
FIG. 2B shows the current-voltage characteristic of a protection circuit such as shown in FIG. 2A. This drawing illustrates the shape of current It through thyristors Th1 and Th2 which are conducting according to the voltage Vt across them. The right portion of the curve corresponds to the action of thyristor Th1 and diode Z1, the left portion of the curve corresponding to the action of thyristor Th2 and diode Z2.
The protection device 7 between the SLIC and circuit 2 has a structure similar to that of device 6, but with different and asymmetrical triggering thresholds, for example, 0 and -50 volts. It should be noted that in the speech mode, device 6 is not necessary since device 7 triggers first.
A disadvantage of a system such as that shown in FIG. 1 is that the cost of the protection is high since it requires at least two protection devices to be supplied and mounted separately.
Another disadvantage of such a system is that, if an overvoltage occurs as relays I1 and I2 are connecting the line to SLIC circuit 1, protection device 7 will start operating. However, for the time period of the overload, a strong current flows through the contacts of relays I1 and I2, thereby risking to damage them and making it impossible to use static relays.